The Role of Nonlinear Drying above the Boundary Layer in the Mid-Holocene African Monsoon

Abstract

Paleoclimatic proxies indicate that significant summertime rainfall reached the Sahara region during the mid-Holocene, presumably in response to stronger summertime heating in the Northern Hemisphere. Climate models generally do not replicate the enhanced precipitation. As a step toward understanding the response and possible role of model errors, a series of idealized experiments were conducted with the Community Earth System Model in which local atmospheric heat sources of increasing magnitude were applied in the boundary layer over the Sahel and Sahara. In response to this local heating, the cold and moist southwesterly monsoon inflow encroaches farther northward. A source strength of roughly 1 K day−1 produces responses similar to those in a simulation with mid-Holocene orbital forcing imposed globally, while that of 1.5 K day−1 produces a precipitation response similar to that from paleoproxies. The precipitation increases nonlinearly, with a jump at heating of around 1 K day−1, even though the low-level monsoon inflow increases linearly. Competition at low-to-middle levels between drying by a shallow return flow just above the boundary layer and moistening by vertical advection within the layer affects convection and determines the northward extension of precipitation. When the heating becomes 1.5 K day−1, the boundary layer flow encroaches sufficiently northward to weaken the shallow return flow, further aiding precipitation. This novel nonlinear mechanism operates without biogeophysical feedbacks, and suggests that poor representation of the local thermodynamic processes may hamper a model’s ability to simulate dynamical feedbacks and hence the strength and poleward extension of monsoon rains under forcings like those during the mid-Holocene.

Influence of the representation of convection on the mid-Holocene West African Monsoon

“…As another factor, changes in dust fluxes between present-day and mid-Holocene conditions have been mentioned and are a subject of controversy (Pausata et al, 2016;Thompson et al, 2019). Finally, by adding an artificial heating source within the atmospheric boundary layer over the Sahara, Dixit et al (2018) were able to increase the magnitude and northward extent of precipitation comparable to what is seen in proxy data. They argued that GCMs miss an important local diabatic heating source over the Sahel-Saharan region.…”

Influence of the representation of convection on the mid-Holocene West African Monsoon

“…As another factor, changes in dust fluxes between present-day and mid-Holocene conditions have been mentioned and are a subject of controversy (Pausata et al, 2016;Thompson et al, 2019). Finally, by adding an artificial heating source within the atmospheric boundary layer over the Sahara, Dixit et al (2018) were able to increase the magnitude and northward extent of precipitation comparable to what is seen in proxy data. They argued that GCMs miss an important local diabatic heating source over the Sahel-Saharan region.…”

Mechanisms and Impacts of Earth System Tipping Elements

“…This does not mean that atmospheric moist dynamics are incapable of causing nonlinear, abrupt changes: Dixit et al. (2018) used idealized heatings in a global climate model to argue that dry air advection from the Sahara suppresses West African rainfall in the modern climate, and that a northward shift of that rainfall in response to the mid‐Holocene insolation forcing might have shutdown that dry air advection and produced nonlinear intensification of the West African monsoon. Seshadri (2017) found bifurcations indicative of tipping points when varying the parameters of a simple box model of monsoons, but noted that it was important to determine whether those parameter values were realistic.…”